There are four basic steps in the biological manufacture of ethanol. These steps include;                1) The liquefaction of the feed by heat, acid and/or enzymatic digestion to produce a liquefied mash.        2) The saccharification of the liquefied mash in order to produce hydrolyzed starches and sugars.        3) The fermentation of the sugars by yeast to produce ethanol and carbon dioxide        4) The purification and recovery of ethanol by distillation-dehydration.        
A typical fermentation process will employ grains such as corn or sorghum as feedstock. The carbon dioxide evolved from fermentation is often purified, liquefied and distilled for sale as an industrial gas for subsequent use in food preservation, dry ice production, and beverage carbonation.
FIG. 4 depicts the basic unit operations associated with ethanol manufacture.
Ethanol production begins by grinding feedstock 401 such as corn into coarse flour in milling step 402, and combining the ground material 403 with water and enzymes in saccharification step 404 where enzymes 405 that are added convert the feed into a fine slurry, the slurry is heated for sterilization, and the slurry is pumped to a liquefaction tank where other enzymes 407 are added in order to convert the starches into glucose-sugars. The sugars are then combined in step 100 with yeast 10 for fermentation. After fermentation, which produces gaseous byproduct carbon dioxide 30, the “mash” 20, a mixture of solids and water, is filtered in step 110 from which the liquid product 21 is fed to distillation/drying step 120 in which product ethanol stream 22 is obtained. The stream 50 of moist solids obtained in filtering step 110, comprising spent grain, is typically concentrated in step 110 to a 30-45% solids-syrup (WDGS). A portion or all of the WDGS is then typically dried in drying step 408 often by super-heated air 410. The resulting solids product 51, typically referred to as distillers dry grain solids plus solubles (DDGS), is often used as a livestock feed supplement.
It has become known that the manufacture of ethanol in this manner produces and liberates volatile organic compounds (referred to as “VOC”). These VOC compounds are present in the gaseous stream 412 which is produced in drying step 408 in high enough amounts that simple atmospheric venting of stream 412 is environmentally impermissible. Similarly, VOCs are also liberated into carbon dioxide stream 30 during the fermentation process. The costs attributable to mitigation of VOC from fermentation and DDGS manufacture can be substantial. As an example, the operation of a regenerative thermal oxidation (RTO) system for a 30 to 40 million gal/yr ethanol plant may constitute upwards of 5% of the unit cost to produce ethanol. Given the fuel consumption of an RTO (typically natural gas) the mitigation of VOCs represents a considerable ongoing (and increasing) expense.
Thus, there remains a need for effective, economical methods for avoiding atmospheric emissions of contaminants in the treatment of biological (carbohydrate) feed material to produce ethanol.